The production of multilayer printed wiring boards (PWB's) for very high speed integrated circuitry requires laminates that fulfill several requirements. Among these are a low dielectric constant, preferably less than 3.0, low dielectric loss, minimal water absorption, a coefficient of thermal expansion compatible with that of surface mounted devices, and, preferably, processing conditions similar to those used in conventional laminate manufacture. Also important is a high glass transition temperature (Tg), preferably above 185.degree. C., because a high glass transition temperature insures good dimensional stability during use, especially at higher temperatures, so that the laminate does not warp. Also, many properties such as dielectric constant, dissipation factor, and coefficient thermal expansion are adversely affected at the glass transition temperature, so the higher the glass transition temperature is the better. The epoxy resins that are used in conventional PWB's are unsatisfactory because their dielectric constants and losses are too high, and their glass transition temperatures are too low.
High vinyl (i.e., having a high percentage of 1,2-unsaturation) polybutadienes (PBD) are attractive resins for use in circuit board laminates since they exhibit a dielectric constant less than 2.5, a dielectric loss tangent below 10.sup.-4, and have a large amount of pendant unsaturation available for crosslinking. A further benefit is that low molecular weight, low viscosity PBD resins are commercially available, permitting formulations high in nonvolatile content. Polybutadiene does, however, show several deficiencies as a laminating resin, most of which are associated with its cure behavior. The Tg of cross-linked polybutadiene is highly dependent on processing conditions; high Tg's near 200.degree. C. are achieved only by a slow, stepwise cure which is incompatible with commercial B-staging and press curing practices. Additionally, when prepregs made from polybutadiene resins that contain peroxide free-radical initiators are B-staged in the presence of oxygen, they undergo surface oxidation in preference to chain reaction of the pendant vinyl groups. Finally, because of the chain growth cure mechanism and the extremely tacky nature of uncrosslinked PBD, nontacky prepregs that will flow during press cure are generally not attainable.
A number of modified resin systems have been examined with an aim to reducing these limitations. The use of styrene block copolymers improves the B-stage behavior but limits the Tg. Co-curing PBD with a number of acrylates or polyunsaturated monomers (e.g., triallyl cynaurate) results in improved processing but lowers the Tg and raises the dielectric constant. The incorporation of bismaleimides into polybutadiene resins has been described, but, in general, bismaleimides are not compatible with polybutadiene, and the cured laminates reflects two-phase behavior by displaying separate mechanical relaxation transitions for each phase. Alloys of polybutadiene with bismaleimide-triazine prepolymers yield high Tg laminates. However, the dielectric constant is 3.2 or greater for a quartz based laminate and phase separation occurs at all but a relatively narrow range of resin compositions.